Method and device for determining and compensating for the tilting of the spectrum in an optical fiber of a data transmission path

ABSTRACT

A method is provided for determining and setting the tilting of the spectrum of light signals in an optical fiber of an optical data transmission path having at least one part for varying the tilting of the spectrum, wherein the light signals are amplified by at least one optical amplifier and a portion of the amplified light signals is extracted, the extracted light signals are then partially guided through an influencing element with a known frequency-dependent intensity influence, the influencing element being an amplifier, a waveguide structure or a fiber with an amplifying action, the total intensity of the extracted light signals is then measured upstream and downstream of the influencing element prior to the extracted light signals being guided through the influencing element, and the control criterion is determined, based on the known frequency-dependent intensity influence of the influencing element and the measured total intensity, for setting the tilting via which the part for varying the tilting is controlled.

[0001] The invention relates to a method and a device for determiningthe tilting of the spectrum in an optical fiber of a data transmissionpath. The invention also relates to a method and a device forcompensating for the measured tilting.

[0002] It is known that power is transferred from higher to lowerfrequencies (from lower to higher wavelengths) and thus between datatransmission channels in optical fibers by stimulated Raman scattering(SRS) . Thus, the original frequency spectrum of the light signal is“tilted”. This reduces the received power of the channels with shortwavelengths, thus increasing their bit error rate. It is also known tomeasure the tilting of the spectrum of light signals that are guidedthrough optical fibers, in particular of optical data transmissionpaths, and to counteract this tilting by appropriate filtering oramplification.

[0003] In order to determine this tilting, use is made in the prior artof a complicated spectrally resolving measuring technique that cannot bewidely applied because of the expensive and also bulky measurementtechnology.

[0004] Furthermore, there is known from US patent US 5,818,629 a methodand an arrangement for determining a mean wavelength (“momentalwavelength”) of the transmitted light signals, and a control, dependentthereon, of an optical amplifier for compensating for the tilting of thespectrum of the transmitted light signals, in the case of which the“momental wavelength” of the injected light signals is determined in a“monitoring device” (see FIG. 1). The mean wavelength (“momentalwavelength”) is used to determine the tilting of the spectrum of theinjected light signals within the optical transmission system, and tocontrol the amplification of the optical amplifier so as virtually tocompensate for the determined tilting of the spectrum. Such a methodrequires the technically complicated determination of the meanwavelength and the evaluation thereof.

[0005] It is therefore the object of the invention to develop a methodand a device that can determine the tilting of the spectrum in anoptical fiber in a simple and quick way without a spectrally resolvingmeasuring technique.

[0006] This object is achieved by means of the two independent patentclaims.

[0007] The inventors have made the following findings:

[0008] As a consequence of the stimulated Raman scattering (SRS), poweris transferred from channels with shorter wavelengths to channels withlonger wavelengths. Some of the channels thus experience an additionalattenuation, while the others experience through this nonlinear effectan amplification counteracting the fiber attenuation. This amplificationor additional attenuation is a function of time. However, this aspectcan be neglected in the case of strongly differing group delays betweenthe interacting channels, which is frequently the case when use is madeof SSMF (Standard Single Mode Fiber). Nevertheless, the result isprecisely different mean powers for the individual channels as from thewavelength dependence of the gain in EDFA (Erbium Doped FiberAmplifiers). This effect is termed “tilting” of the spectrum. It ispossible in principle for the gain of an EDFA to be set specificallysuch that this effect is countered. However, compensating for anytilting in a data transmission path requires a simple method to be foundfor determining the tilting.

[0009] It is possible in principle to determine a linear tilting, thatis to say a first-order tilting, by means of the information from twototal intensities in the spectrum respectively after the passage throughat least one filter with a known frequency-dependent transmissioncharacteristic, or at least one amplifier with a knownfrequency-dependent influencing characteristic, designated in general asinfluencing element below. Tiltings of higher order, that is to saynonlinear tiltings, can be approximated correspondingly by anappropriately high number of measurements of total intensities after thepassage through other known frequency-dependent influencing elements ineach case.

[0010] In order to determine the spectral tilting of a signal, it issufficient in principle to extract light at a site and to determine thetotal intensity after the passage of two influencing elements—forexample filters or frequency-dependent amplifiers. One of these filterscan also be reduced to an all-pass filter without phase response, suchthat it can just as well be removed. One influencing element and twomeasuring sites then suffice here for the total intensity, in order todetermine the spectral tilting of the signal. However, if the totalintensity of a signal is already known from other information before thepassage through an influencing element, a single influencing element anda single measuring site of the total intensity downstream of thisinfluencing element also suffice.

[0011] Signal tiltings in a signal path and tilting caused by the EDFAare fundamentally similar if a flat input spectrum is presupposed.However, in a transmission system the spectrum at the transmitter end isdeliberately predistorted such that the signal tilting downstream andupstream of the EDFA must be determined in order to determine thetilting by the EDFA, since otherwise the information about the inputsignal is not to hand.

[0012] In accordance with these inventive ideas outlined above, theinventors propose to improve the known method for determining thetilting of the spectrum of light signals in an optical fiber of anoptical data transmission path by virtue of the fact that the opticaldata transmission path has at least one means for varying the tilting ofthe spectrum, and the light signals are amplified by at least oneoptical amplifier, and a portion of the amplified light signals isextracted. The extracted light signals are partially guided through aninfluencing element with a known frequency-dependent intensityinfluence. Furthermore, the total intensity of the extracted lightsignals is measured upstream and downstream of the influencing element,and the total intensity of the light signals is measured before theamplification. Use is made in this case as influencing element (11) ofan amplifier or a waveguide structure or fiber with an amplifyingaction. There is determined on the basis of the known influence of theinfluencing element (11) and the measured total intensities a controlcriterion for setting the tilting via which the means for varying thetilting is controlled.

[0013] In a particular refinement of this method, it is provided thatuse is made as influencing element of a settable optical filter and/or afrequency-dependent amplifier, it preferably being possible for this tobe an EDFA (Erbium-doped Fiber Amplifier). It is also possible to makeuse as amplifier of other waveguide structures doped with rare earths. AMach-Zehnder with adjustable time delay in one branch or settableintensity division onto the two branches can be named as an example fora settable optical filter.

[0014] In accordance with the idea of the invention, the measuringmethod represented above can also be used for a method for setting orcompensating for the tilting of the spectrum of light signals in anoptical fiber of an optical data transmission path. This tilting can bevaried or compensated for by virtue of the fact that one or moresettable filters or attenuators and/or the frequency dependence of theamplification of one or more optical amplifiers, for example EDFA orother waveguide structures doped with rare earths are set in such a waythat they counteract the tilting produced on the transmission path.

[0015] It can be provided here according to the invention that the meansfor varying the spectrum is a frequency-dependent optical amplifier,preferably a waveguide structure doped with rare earths, a fiber or anEDFA, it being possible for the frequency dependence of theamplification of the waveguide structure or fiber to be set by varyingthe pump power in such a way that it opposes the original tilting.

[0016] Further features of the invention emerge from the subclaims andthe following description of the exemplary embodiments with reference tothe drawings.

[0017]FIG. 1a shows how the SRS influence can be reduced by tilting thegain of an EDFA, the transmission characteristic of the filter usedbeing illustrated in FIG. 1b therebelow.

[0018] Since optical data transmission paths can be of very differentdesign, and the spectral power distribution can change during operation,only a variable, that is to say settable, “gain tilting” makes sense. Itis assumed in the following considerations that the mean populationinversion of an EDFA in the initial state is selected such that minimaldifferences in gain—without the use of a filter - occur that are furtherlargely completely eliminated with the aid of a filter.

[0019] In order to counteract the effect of the SRS, channels mustexperience a greater amplification for shorter wavelengths than forlonger wavelengths. Precisely this effect occurs when the meanpopulation inversion of the EDFA is increased, which is illustrated inFIGS. 1a and 1 b. The gain profile is completely flat in the initialstate. In order to achieve this, a filter with the transmissioncharacteristic 3 shown in FIG. 1b was adopted. If the pump power in oneor both amplifier stages 7 is now increased in the case of an EDFAdesign as shown in FIG. 3, the mean population inversion increases andthe desired gain tilting occurs. Such a possible gain tilting as afunction of the set power is also illustrated in the spectra of FIG. 4.

[0020] As FIGS. 1a and 1 b show, however, this compensation methodcauses two difficulties. If the power of the equidistant channels isplotted logarithmically against their wavelength, the straight line 1 isyielded to a very good approximation when only the SRS is acting. Theprofile of the gain 2 of an EDFA does not, however, exhibit a linearprofile as a function of the wavelength, and so no complete compensationis possible. In the example shown for an EDFA with 30 dB gain in theinitial state, the pump power was set so as to result in a difference ingain 4 of at most 3 dB. Because of the nonideal shape of the curve,power differences 5 of 0.9 dB occur after the action of the SRS betweenthe channels. This deviation from the ideal shape of the curve can,however, be compensated for by inserting appropriate filters into thepath at a few points. In some circumstances, even the setting ofdifferent transmit powers already suffices to obtain equal power levelsand/or signal-to-noise power ratios at all the receivers. A furtherdisadvantage of the method is that the mean gain likewise increases.This can be compensated for by increasing the attenuation of theinserted attenuator. The error turns out to be substantially smallerwhen the channels are displaced to higher wavelengths by approximately10 nm. The starting point in the example was a wavelength range of 1570nm to 1605 nm, the so-called L band. However, the method can also beapplied for other wavelength ranges.

[0021] The increase in the mean gain is illustrated in FIG. 2a, and the“error” occurring is illustrated in FIG. 2b as a function of thetilting. It may be seen that the power differences occurring owing tothe SRS can be compensated for to approximately ⅔. In the initial state,the gain in decibels is${G_{opt} = {\frac{10}{\ln \quad 10}{L \cdot \left\{ {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot {\overset{\_}{N}}_{opt}} - {\sigma^{a}(\lambda)}} \right\}}}},$

[0022] L standing for the total length of the doped fiber, σ^(e)(λ) andσ^(a)(λ) representing the coefficients, dependent on the wavelength λ,for emission and absorption, respectively, and {overscore (N)}_(opt)representing the mean population inversion in the initial state.

[0023] It is assumed below that the differences in gain are completelycompensated for in the initial state with the aid of a filter. If themean population inversion is now increased by the value Δ{overscore(N)}, the result for the gain is$G_{comp} = {\frac{10}{\ln \quad 10}{L \cdot {\left\{ {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot {\overset{\_}{N}}_{opt}} + {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot \Delta}\quad \overset{\_}{N}} - {\sigma^{a}(\lambda)}} \right\}.}}}$

[0024] By comparison with the initial state, an increase in gain by${\Delta \quad G_{comp}} = {\frac{10}{\ln \quad 10}{L \cdot \left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot \Delta}\overset{\_}{N}}$

[0025] is thus to be recorded. The gain tilting effected by an increasein the mean population inversion can therefore be described by afunction f(λ) that is fixed by the active cross section and is still tobe multiplied by a factor. The last equation above makes it plain thatthe compensation of the SRS cannot be improved when the starting pointis another initial state of the EDFA.

[0026] In order to be able to use this method in a commercialtransmission system, it is necessary to have available a suitable rulethat can be implemented easily.

[0027] As already set forth, a unique relationship exists between theincrease in the internal gain and the tilting. The internal gain can bedetermined from the gain measured between input and output, by furtheradding the attenuation of an inserted attenuator. A controlled variablefor the tilting is indirectly obtained thereby.

[0028] A grave difficulty results, nevertheless. Since no measuringdevice is generally available for spectrally resolved measurement, onlythe total power is known at the input and output of the amplifier, butnot how this is distributed over the individual channels. It is thusimpossible to determine a mean gain as unique reference variable. Asimple solution to this problem comes from the extended EDFA designshown in FIG. 3. Here, it is not only the total power that is measuredat the input and at the output of the amplifier, but also the power at aspecific wavelength. It is therefore possible to determine uniquely thegain for this wavelength channel, and thus also the tilting.

[0029] In order to reduce the influence of nonlinear fiber effects,various methods can be applied for occupying the available wavelengthrange depending on the type of fiber used. The above-described design ofthe invention leads to restrictions, since the wavelength channel usedto measure the gain must always be in operation.

[0030] A possible solution that circumvents this restriction can be setforth, as illustrated in FIG. 3.

[0031] This FIG. 3 shows a two-stage amplifier comprising a datatransmission path with controllable gain tilting intended forcompensating for the tilting caused by stimulated Raman scattering(SRS), with two controllable amplifier stages (EDFA) 7. For the purposeof clarity, the associated electronic arrangement is not illustrated.Upstream of the first amplifier stage 7 a component signal is extractedvia the first coupler 6, and a first photodiode 10 is used to measurethe unfiltered total intensity and, after filtering by the knownfrequency-dependent filter 11, likewise to measure the filtered totalintensity. In accordance with the following description, these data areused to determine the input tilting of the signal into the firstamplifier stage 7. Located downstream of the first amplifier stage 7 isa further coupler 6 and a photodiode 11 for controlling the gain of thefirst amplifier stage 7 in cooperation with the total intensity measuredat the input. Subsequently, the signal passes a settablefrequency-independent attenuator 8, a further coupler 6, which in turnextracts a component signal at the input of the second amplifier stage 7and feeds it unfiltered to a photodiode 10 for measurement. Downstreamof the second and last amplifier stage 7, once again, the resultingsignal is partially extracted and fed for measurement to a measuringarrangement 9 with a photodiode 10 without prefilter and a photodiode 10with an upstream filter 11. The tilting of the signal exiting theamplifier arrangement is determined in the way according to theinvention by the last measuring arrangement, and the tilting iscorrespondingly kept within the desired bounds or completely compensatedfor by varying the inversion of the EDFA, that is to say by controllingthe gain of the amplifier stages 7. The settable attenuator 8 serves apurpose in this case of reducing the gain of the total amplifier, ifappropriate in a frequency-independent fashion, or of raising it byretracting a preset attenuation.

[0032] Thus, in this design it is not the task of the illustrated filter11 to select an individual channel, but to simulate in its attenuationresponse the wavelength dependence of the gain tilting, that is to saythe function f(λ) except for a constant of proportionality. Itstransmission characteristic is

T(λ)=exp{−α·ƒ(λ)},

[0033] in which the constant α may be known. If the powers of the Nchannels are designated by P_(i) and their wavelengths by λ_(i), thepowers measured at the input are$P_{in} = {\sum\limits_{i = 1}^{N}\quad P_{i}}$

[0034] and, after filtering,$P_{in}^{filt} = {\sum\limits_{i = 1}^{N}\quad {{P_{i} \cdot \exp}{\left\{ {{- \alpha} \cdot {f\left( \lambda_{i} \right)}} \right\}.}}}$

[0035] It holds correspondingly for the powers measured at the output ofthe amplifier that$P_{out} = {G_{opt} \cdot {\sum\limits_{i = 1}^{N}\quad {P_{i}\exp \left\{ {\chi \cdot {f\left( \lambda_{i} \right)}} \right\}}}}$

[0036] and, after filtering,$P_{out}^{filt} = {G_{opt} \cdot {\sum\limits_{i = 1}^{N}\quad {P_{i}\exp {\left\{ {\left( {\chi - \alpha} \right) \cdot {f\left( \lambda_{i} \right)}} \right\}.}}}}$

[0037] The constant χ determines the degree of gain tilting and is to bedetermined below. The first step for this purpose is to expand theexponential function in a series and truncate it after the second-orderterm. This results in the system of equations $\begin{matrix}{{\frac{P_{out}}{G_{opt}} - P_{in}} = {{\chi \cdot {\sum\limits_{i = 1}^{N}\quad {{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\chi^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\quad {{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}} \\{{\frac{P_{out}^{filt}}{G_{opt}} - P_{in}} = {{\left( {\chi - \alpha} \right) \cdot {\sum\limits_{i = 1}^{N}\quad {{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\left( {\chi - \alpha} \right)^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\quad {{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}} \\{{P_{in}^{filt} - P_{in}} = {{{- \alpha} \cdot {\sum\limits_{i = 1}^{N}\quad {{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\alpha^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\quad {{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}}\end{matrix}$

[0038] consisting of three equations in which the three unknowns$\chi,{\sum\limits_{i = 1}^{N}\quad {{{f\left( {\lambda \quad i} \right)} \cdot P_{i}}\quad {and}\quad {\sum\limits_{i = 1}^{N}\quad {{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}$

[0039] are obtained. The gain G_(opt) in the initial state is known fromthe design and dimensioning of the EDFA. It is therefore possible todetermine the target variable χ uniquely:$\chi = {\alpha \cdot {\frac{P_{out} + P_{out}^{filt} - {\left( {P_{in} + P_{in}^{filt}} \right) \cdot G_{opt}}}{P_{out} - P_{out}^{filt} + {\left( {P_{in} - P_{in}^{filt}} \right) \cdot G_{opt}}}.}}$

[0040] The series expansion was terminated after the second-order termin order to keep the outlay low. The exponential function can thereforebe approximated only within a bounded value range. If this value rangeis to be enlarged, terms of higher order can likewise be taken intoaccount, there being a need to use further photodiodes with differentupstream filters.

[0041] The Taylor series expansion yields a very good approximation ofthe exponential function for very small arguments, while greaterdeviations occur in the case of greater values. Consequently, thesuggestion is to adapt the factor ½ in front of the second term suchthat the maximum error occurring becomes minimal within the desiredvalue range. If the factor ½ is replaced, for example, by 0.81, gaintiltings of up to 4.5 dB can be set in conjunction with a maximum errorof 0.18 dB.

[0042] In accordance with a further reaching aspect according to theinvention, which leads to a particularly simple and elegant device fordetermining the tilting of the frequency spectrum, the following ideamay be represented still special application of the abovedescribedprinciple:

[0043] When considering the tilting in the case of a measured spectrumS(λ) in the wavelength region of λ_(start) to λ_(stop), it is possibleto determine it by a numerical analysis, and to characterize acharacteristic quantity for the tilting, for example the first moment M₁of the spectrum relative to the middle wavelength λ_(c) of the spectrum(λ_(c)=(λ_(start)+λ_(stop))/2):M₁ = ∫_(−L/2)^(+L/2)xS(x + λ_(c))  x  with  L = (λ_(Stop) − λ_(Start))

[0044] It is also possible to use other odd functions f(x) (here, oddmeans that f(x)=−f(−x)): V = ∫_(−L/2)^(+L/2)f(x)S(x − λ_(c))  x

[0045] According to the invention, instead of a complicated spectrallyresolved measurement of the spectrum S(λ) and a subsequent numericaldetermination, by spectral analysis, of the tilting, the spectrum isweighted with the frequency response G(λ) with the aid of an opticalfilter, and the aggregate output power P_(v) of the filter is measuredwith the aid of a simple photodiode. The weighting can be adapted inthis case to the expected tilting: P_(V) = ∫₀^(+∞)G(λ)S(λ)  λ

[0046] Since the frequency response G(λ) and the spectrum S(λ) aregreater than 0, P_(v) is also greater than 0 even in the case of anuntilted spectrum. This offset is to be borne in mind during use.Furthermore, the frequency response G(λ) from: λ_(start) to λ_(stop)should be odd in relation to G(λ_(c)) (here, odd means thatG(λ_(c)+x)−G(λ_(c))=−[G(λ_(c)−x)−G(λ_(c))]). Moreover, the monotonicedge of the filter frequency response should extend from λ_(start) toλ_(stop). Again, frequency responses of photodiodes or couplers can betaken into account in G(λ), if they would otherwise lead tofalsifications of the measurement result. A bandpass restriction to awavelength region to be considered (for example from λ_(start) toλ_(stop)) can likewise be included in G(λ).

[0047] A plurality of linearly tilted spectra S(λ)=a·λ+b, illustrated asdashed lines, are shown as a functional example in FIG. 4. The totalpower ∫₀^(+∞)S(λ)  λ

[0048] of all the spectra is the same here, and 0 so the same powerwould be measured by means of a photodiode independently of the tilting.If a filter with frequency response G(λ) is now inserted, the powermeasured at the photodiode becomes dependent on the tilting, asillustrated in FIG. 5, and it is thereby possible to use it as a measureof the tilting for control tasks.

[0049] For example, a Mach-Zehnder interferometer with a cos 2-typefrequency response can be used as suitable filter. In this case, themeasured value has an offset dependent on the aggregate power of theoptical signal. Offset means here that the measuring device supplies asignal even given a vanishing tilting of the spectrum. This disadvantagecan be avoided by means of an optical filter with two opposing frequencyresponses G_(AB)(λ) and G_(AC)(λ) and if it holds thatG_(AB)(λ)+G_(AC)(λ)=const. An example of implementation with twoopposing frequency responses is the use of the two outputs of aMach-Zehnder interferometer. The tilting and the total power of thesignal can be determined simultaneously with the aid of this design:

[0050] The tilting V is yielded from the difference between the measuredvalues by:V = P_(VC) − P_(V  A) = ∫₀^(+∞)[G_(A  C)(λ) − G_(A  B)(λ)]S(λ)λ

[0051] The offset of V vanishes in the case ofG_(AB)(λ_(c))=G_(AC)(λ_(c)) for a linear frequency response.

[0052] The aggregate power P is calculated from the sum of the measuredvalues:P = P_(VC) + P_(V  A) = ∫₀^(+∞)[G_(A  C)(λ) + G_(A  B)(λ)]S(λ)λ = const.∫₀^(+∞)S(λ)  λ

[0053] This variable is advantageously used as early as when controllingthe pump laser diodes in fiber amplifiers, and can now also be used fornormalizing the tilting if the magnitude of the tilting and not only thesign is required.

[0054] The very simple and therefore cost-effective design of thismeasurement proves to be particularly advantageous in this solutionillustrated, there being no need for spectrally resolved measurement. Adecentral control becomes possible, as a result of which the outlay oncontrol software is reduced and the control rate is increased.Furthermore, the weighting can be adapted to a fundamentally knowntilting function of the fiber amplifier, and to possible disturbances inthe spectrum such as, for example, tilting owing to SRS attenuation.

[0055] It goes without saying that the abovenamed features of theinvention can be used not only in the combination respectivelyspecified, but also in other combinations or on their own, withoutdeparting from the scope of the invention.

[0056] Thus, as a whole this invention exhibits a simple method and adevice for determining the tilting of the spectrum of an optical signalby measuring at least one total intensity subsequent to a passage of thesignal through an influencing element with a known influencingcharacteristic, including the possibility of using it to set thespectral tilting.

1. A method for determining the tilting of the spectrum of light signals in an optical fiber of an optical data transmission path having at least one means for varying the tilting of the spectrum, in which the light signals are amplified by at least one optical amplifier (7) and a portion of the amplified light signals is extracted, in which the extracted light signals are partially guided through an influencing element (11) with a known frequency-dependent intensity influence, in which the total intensity of the extracted light signals is measured upstream and downstream of the influencing element (11), characterized in that use is made as influencing element (11) of an amplifier or a waveguide structure or fiber with an amplifying action, is that the total intensity of the light signals is measured before the amplification, and in that there is determined on the basis of the known influence of the influencing element (11) and the measured total intensities a control criterion for setting the tilting via which the means for varying the tilting is controlled.
 2. The method as claimed in claim 1, characterized in that use is made as influencing element of a filter (11), preferably a Mach-Zehnder interferometer or a dielectric filter or a fiber grating or a wavelength-selective coupler, in particular a fusion coupler.
 3. The method as claimed in claim 1 or 2, characterized in that at least one influencing element of an amplifier (7) is an EDFA (Eribium-doped Fiber Amplifier).
 4. The method as claimed claim 1, characterized in that use is made of at least one optical amplifier (7) as means for varying the tilting.
 5. The method as claimed in claim 1, characterized in that use is made of a settable frequency dependent optical filter as means for varying the tilting.
 6. The method as claimed in claim 5, characterized in that the settable frequency-dependent optical filter used is a Mach-Zehnder interferometer with adjustable power division into its two branches or with settable time delay in the at least one of its two branches.
 7. The method as claimed in one of claims 4 to 6, characterized in that an existing tilting is compensated for competely or partially or is set to a tilting in the opposite direction.
 8. The method as claimed in one of claims 4 to 7, characterized in that a tilting of a predefined magnitude is produced. 